Charge transport in a polar metal

Abstract

The fate of electric dipoles inside a Fermi sea is an old issue, yet poorly-explored. Sr1−xCaxTiO3 hosts a robust but dilute ferroelectricity in a narrow (0.002 < x < 0.02) window of substitution. This insulator becomes metallic by removal of a tiny fraction of its oxygen atoms. Here, we present a detailed study of low-temperature charge transport in Sr1−xCaxTiO 3−δ , documenting the evolution of resistivity with increasing carrier concentration (n). Below a threshold carrier concentration, n * (x), the polar structural phase transition has a clear signature in resistivity and Ca substitution significantly reduces the 2 K mobility at a given carrier density. For three different Ca concentrations, we find that the phase transition fades away when one mobile electron is introduced for about 7.9±0.6 dipoles. This threshold corresponds to the expected peak in anti-ferroelectric coupling mediated by a diplolar counterpart of Ruderman-Kittel-Kasuya-Yosida (RKKY) interaction. Our results imply that the transition is driven by dipole-dipole interaction, even in presence of a dilute Fermi sea. At higher carrier concentrations, our data resolves slight upturns in low-temperature resistivity in both Ca-free and Ca-substituted samples, reminiscent of Kondo effect and most probably due to oxygen vacancies.arXiv:1909.04278v1 [cond-mat.supr-con]

Ferroelectric Exchange Bias Affects Interfacial Electronic States

“…We note that when 3D bulk conductivity is induced in Ca-doped STO by the creation of oxygen vacancies (obviously without any intrinsic bias), only a small change in the resistance is observed below the ferroelectric transition temperature, [24,43] further demonstrating the importance of the intrinsic bias and current confinement for observing such a large effect. Ferroelectric control of the LAO/STO interface was also demonstrated by polarizing a ferroelectric layer deposited on the LAO.…”

A large modulation of electron-phonon coupling and an emergent superconducting dome in doped strong ferroelectrics

“…On the other hand, the superconductivity in doped SrTiO 3 has drawn great interests from both theorists [8][9][10][11][12][13][14][15] and experimentalists [16][17][18][19][20][21][22][23][24] . One beautiful experiment is Sr 1−x Ca x TiO 3−δ in which Ca doping leads to a weak ferroelectric distortion in SrTiO 3 and oxygen vacancies provide itinerant electrons 20,25,26 . Increasing the carrier concentration in Sr 1−x Ca x TiO 3−δ induces a polar-to-centrosymmetric phase transition and a superconducting "dome" emerges around the critical concentration.…”

Ferroelectricity, Superconductivity, and SrTiO3—Passions of K.A. Müller

“…For all investigated samples, the resistance shows a small upturn below ∼15 K, the origin of which is still under investigation. It should be noted that an anomaly/upturn has been observed in the low-temperature resistivity of bulk Ca-substituted SrTiO 3−δ samples and was associated to a ferroelectric-like state still existing in metallic samples [46,67]. Similarly, a study performed on heterostructures of LaAlO 3 grown on top of Ca-substituted SrTiO 3 substrates showed the presence of a resistance upturn occurring just below the Curie temperature, possibly linked to the ferroelectricity of the substrate [68].…”